Conventionally, as a doubly rotated crystal resonator, an SC cut crystal resonator is known as shown for example in FIG. 3, in which electrodes are formed on a quartz crystal blank 1 cut from a surface of a quartz crystal where the surface of the quartz crystal orthogonal to a Y axis is rotated through approximately 33° about the X axis and then from this rotated position it is rotated through approximately 22° about the Z axis (for example, refer to Japanese Unexamined Patent Publication (TOKKYO KOKAI) No. Hei 11-177376).
This SC cut crystal resonator has temperature characteristics expressed in a cubic curve similar to that of an AT cut, and its inflection point is in the vicinity of 94° C. It is used as a crystal resonator of a so-called oven controlled crystal oscillator, in which a crystal resonator is temperature controlled.
However, as shown in FIG. 4, in the SC cut crystal resonator, secondary vibration of B mode (denoted by B in FIG. 4) and A mode (denoted by A) vibration occurs in the vicinity of a C mode resonance (vibration) (denoted by C), which is a main vibration, and is on the high pass side thereof.
Here, since the value of an equivalent resistance (hereunder referred to as “CI” (Crystal Impedance)) of the A mode vibration is greater than a CI of the C mode, then as an oscillator, the signal cannot be easily output. Therefore particularly in a crystal resonator this does not become a problem.
On the other hand, the CI of the B mode vibration is equal to the CI of the C mode vibration or may be smaller in some cases. Furthermore, their frequencies are close to each other, with that of the B mode being approximately 1.09 times that of C mode.
Therefore when the oscillator is actually manufactured, occasionally this oscillates in the B mode, which is a secondary vibration.
Thus, in the case of using an SC cut crystal resonator, it is essential to be able to suppress the B mode vibration in order to reliably excite the C mode vibration, so that it is necessary to make the CI of the B mode larger than the CI of the C mode. Therefore suppression of the B mode vibration is performed by adding mass to the plate surface of the quartz crystal blank, or by designing the holding position thereof, to suppress the spurious oscillation.
However, in order to sufficiently suppress the B mode vibration, a continuing process of trial and error is necessary, which and takes a lot of work, and reproducibility of the process is extremely poor.
The SC cut crystal resonator of the present invention has been achieved in consideration of the problems of such a conventional SC cut crystal resonator, with an object of providing an SC cut crystal resonator wherein, by cut-processing an outer shape of a quartz crystal blank into a special shape, resonance (vibration) due to the B mode can be reliably suppressed with excellent reproducibility.